Optical scanning apparatus

ABSTRACT

An object to be studied or analyzed is flooded with light from a laser. The light field transmitted from the object, which might for example be a transparency or a vibrating surface, is received by a photodetector. Another portion of the laser light or radiation is deflected throughout a scanning pattern and also caused to fall upon the photodetector. An optical system included in the reference beam path brings the reference beam to a focus (either real or virtual image) at a selected distance from the active surface of the photodetector. As a result, the electric signals developed by the photodetector represent a cross-section of the light field, transmitted from the object, located a distance from the photodetector surface which is the same as the distance therefrom to the focus of the scanned reference beam. Different cross-sections of the signal field from the object may be displayed merely by modifying the optical system. Depending on the manner in which the electric signals are processed, they may be caused to represent either a direct image or a hologram of the cross-section being visualized. By using the electric signals to drive a television-type monitor, a real-time display is reproduced.

United States Patent Korpel 51 June 13, 1972 OPTICAL SCANNING APPARATUSABSTRACT [72] Inventor: Adrianus Kor el, Pro fi s, [1] An object to bestudied or analyzed is flooded with light from a laser. The light fieldtransmitted from the object, which [73] Asslgnee zen'th Rad", cm'pommntChlcagoi might for example be a transparency or a vibrating surface, is[22] Filed; March 9 1970 received by a photodetector. Another portion ofthe laser light or radiation is deflected throughout a scanning patternand PP 17,545 also caused to fall upon the photodetector. An opticalsystem included in the reference beam path brings the reference beam toa focus (either real or virtual image) at a selected 52 U.S.Cl.. E IntCl gbii g distance from the active surface of the photodetector. As a Iresult, the electric signals developed by the photodetector [58] Fieldof Search ..73/67.5 H. 340/5 H, represent a cross section of the lightfield, transmitted from the ob'ect, located a distance from thehotodetector surface J P l 5 6] References Cited which is the same asthe distance therefrom to the focus of the scanned reference beam.Different cross-sections of the signal OTHER PUBLICATIONS An OpticalHeterodyne Ultrasonic Image Converter, G. A. Masser, Proceedings IEEEVol. 56, No. 12, Dec. 1968 2157- 2161 Rapid Sampling of AcousticalHolograms Laser-Scanning, Korpel et al., JASA Vol.45, No.4, 1968 PrimaryExaminer-Robert L. Griffin Assistant Examiner-Barry LeibowitzAttorney-John Jr Pederson and Francis W. Crotty field from the objectmay be displayed merely by modifying the optical system. Depending onthe manner in which the electric signals are processed, they may becaused to represent either a direct image or a hologram of thecross-section being visualized. By using the electric signals to drive atelevisiontype monitor, a real-time display is reproduced.

11 Claims, 3 Drawing Figures Q I4 l5 I8 LT TT 17 30 I l 4 r" t "f P f bo, r Laser er 9 Photo- 0 V obleci Detector q Amp 20 Scanning Beam 26 2oScanner o fsu) 8i 2'J\V Shifter 24- 26 27 3\| f (t f(t)+f- 4(1 5 Adder 5Mixer 23w 5 v 33 t lFSignaI q Sweep TV Generator 34 38 Frequency IFGenerator Display Amp 37 Sync Pulse 1 f T W I Generator DetectorP'A'TENTEDJun 12 m2 3. 670.098

' W' g' 1 14 15 as LTTT'J l6 30 II :3 l2 i 1: i 1 Loser 1 Perrurbing 7 52 VJ Phofo- A f v Object Detec'ror Scanning Beam 26 f (t) scorlaner 5 27Sh n 22/ 1 er 25 26 fs(r) s(r)+ i 23 f8) Adder 9* Mlxer W 33 l; IFSignolY Sweep Generoror Frequency T 32 i IF Generoror K P V -39 Amp.

37 Sync Pulse 1 f t Generator Detector t FIG. 2

Photo Detector om Inventor Scanner Adrlonus Korpel a Shifter By M 24Attorney OPTICAL SCANNING APPARATUS The present invention pertains toapparatus for analyzing an object. More particularly, it relates toapparatus utilizing coherent light for permitting reproduction orrecording of an image or a hologram of an object under study.

The introduction of the laser has resulted in the development of anumber of interesting and useful systems for reproducing images ofobjects and for investigating or measuring characteristics of objects.The reproduction of an image refers to a method or system which concernsitself with the amplitude of light that is transitted through orreflected from an object; more precisely, imaging concerns itself withpower which is proportional to the square of the amplitude. Anothertechnique to which great attention has been devoted as a result of theavailability of the laser is that of holography. Holography may bedescribed as a method for reconstructing the amplitude and phasedistribution of a propagating field in a given plane. In the field ofoptics, holography has become identified with three-dimensionalreconstruction. Somewhat analogous techniques have evolved in which asound image is obtained in a way similar to optical image formation andthose techniques have been extended to include holographic systems. Theprinciples and techniques of acoustic imaging have been merged withthose of optical holography, resulting in a new field known as acousticholography.

In optical holography, an image field is made to interfere with aso-called reference beam and the resulting interference pattern isrecorded on photographic, thermoplastic or photochromic film. Thispattern consists of a system of fine fringes varying both in contrastand fringe spacing. The contrast at any particular point is a measure ofthe amplitude of the image field at that point, whereas the positions ofthe fringes relate to the phase with the spacing being determined by theslope of the image field wavefront relative to that of the referencebeam. Thus, although a recording medium is basically responsive only tolight power, it has nevertheless been possible to record both lightamplitude and light phase. Reconstruction of the image field isaccomplished by illuminating the recorded interference pattern with areference beam. Strictly speaking, this generates two related fields(conjugate images) which propagate in different directions and may beseparated by spatial filters. In the analogous systems which have beenused in acoustic holography, a conventional image conversion device isemployed and an acoustic reference beam is added to the sound field. Apattern of fringes appears on the image conversion device. The fringepattern is photographed and the developed negative is illuminated with alaser beam. Depending on the scale of the hologram, differentcross-sections of the sound field may be inspected by various knownmethods.

While electronic camera-type devices such as image orthicons andvidicons have found general use for optical imaging, such devices areinsufficiently sensitive for the inspection or analysis of very weakoptical fields. They also are not sufflciently discriminatory be able todistinguish optical image information represented by light of aparticular frequency that is mixed with other light of slightlydifferent frequencies as is typically the case in acoustic halography.

It is, accordingly, a general object of the present invention to provideapparatus for analyzing or inspecting optical fields which may beextremely weak or require separation from among closely spaced opticalfrequencies.

It is another object of the present invention to provide apparatus whichpermits viewing any desired cross-section of an optical field.

A further object of the present invention is to provide apparatus of theforegoing character which is capable of providing either direct imagingor holographic representation.

One specific object of the present invention is to provide apparatus ofthe foregoing character which enables the reproduction of real-timeacoustic holograms.

Apparatus for analyzing an object in accordance with the presentinvention includes means for flooding a surface area of an object toproduce a coherent-light signal field perturbed by a characteristic ofthe object. Developed concurrent is a reference beam of radiationtime-coherent with the light. The reference beam is deflected throughouta scanning pattern, and a photodetector responds jointly to thereference beam and the signal field to develop electrical signals thatrepresent the object characteristic. In order to view a cross-section ofthe signal field at a preselected distance from the active surface ofthe photodetector, the apparatus further includes an optical system thatbrings the reference beam to a focus at an effective distance from thephotodetector surface corresponding to the pre-selected distance fromthe photodetector surface to the cross-section to be viewed.

The features of the invention which are believed to be novel are setforth with particularity in the appended claims. The invention, togetherwith further objects and advantages thereof, may best be understood,however, by reference to the following description taken in conjunctionwith the accompanying drawings, in the several figures of which likereference numerals identify like elements, and in which:

FIG. 1 is a combined schematic and block diagram of one embodiment ofthe present invention;

FIG. 2 is a diagram of a modification of a portion of the system shownin FIG. I; and

FIG. 3 is a diagram of a modification of a portion of the apparatusshown in either FIGS. 1 or 2.

In FIG. I, a portion of the light from a laser 10 is projected through apartially-silvered mirror 11 to a telescope 12 having an eyepiece l3 andan objective lens 14. Telescope l2 acts on the typically narrow beam oflight from laser 10 to substantially expand its diameter so that itfloods an object 15 which is to be studied, analyzed or reproduced. Asillustrated, the flooding light is transmitted through object 15 afterwhich it passes through another partially-silvered mirror 16 to theactive surface 17a of a photodetector or photo-diode 17. While object 15may be anything capable of in some way perturbing or otherwise alteringthe light passing through it, the simplest example is when object 15 isa transparency having characteristic areas of relative differentattenuation to the light so as to define an image. Thus, the lightquantity l8 emerging from the transparency is an optical signal fieldthat has been perturbed in accordance with the variations in brightnesswhich characterize the image carried on the transparency.

Another portion of the light from laser 10 is reflected downwardly bymirror 11 to form a reference beam 20. Beam 20 is directed by a mirror21 into a scanner and frequency shifter 22. That is, beam 20 isdeflected horizontally and vertically to define a scanning pattern inthe form of a succession of horizontal lines spaced apart in thevertical direction like the scanning of an electron beam in acathode-ray tube to define a television-type image raster. Whilelightbeam deflection may be achieved by the use of mirrors moved under thecontrol of galvanometer-like elements, the system preferably employs anacoustic Bragg deflector arrangement of the kind described an articleentitled A Television Display Using Acoustic Deflection and Modulationof Coherent Light by Korpel et al., in the Oct., 1966 joint issue ofI.E.E.E. PROCEEDINGS, Volume 54, and in APPLIED OPTICS, Volume 5.

As is now well understood, a Bragg deflector propagates acoustic wavesof repetitively varying frequency across the light path. The acousticwaves diffract the beam of light at an angle which is proportional tothe acoustic frequency so that, as the acoustic frequency varies, thelight beam is caused to be deflected or scanned and thus to trace out aline. By using two such acoustic deflectors in series, deflection may beaccomplished in coordinate directions so as to cause beam 20 to traceout a complete raster. As is also now well understood, the diffractionprocess used to deflect beam 20 also effects a shift in the frequency ofthe light emerging from the scanning mechanism. While the acousticfrequency supplied to scanner and shifter 22 is a complicated functionof time in that it varies linearly for both the horizontal and verticaldeflection angles imparted to beam 20, it may be represented for presentpurposes by the expression f,(t) as indicated in FIG. 1 for the signalderived from a sweep frequency generator 23. Representing the light inbeam 20 emerging from laser as having a frequency f,, the deflectedlight leaving scanner and shifter 22 has an instantaneous frequencyf t).

Beyond scanner-shifter 22, reference beam 20 next traverses an opticalsystem 25, in this case simply in the form of a convergent lens 26.After lens 26, beam 20 is reflected by a mirror 27 onto mirror 16 whichin turn reflects a portion of beam 20 onto the active surface ofphotodetector 17. A portion of the light from the diffraction-typescanner, known as the zero order, is not deflected. As shown, thatportion is blocked by a stop 24.

In the photodetector, the light of signal field 18 beats against or ismixed with the light or radiation in reference beam 20. Consequently,the photodetector develops electric signals of the frequency fl,(t) thatinclude modulation components representative of the imagecharacteristics on the transparency. That is, whatever characteristicsof object perturb signal field 18 are represented in the electricsignals. In FIG. 1, lens 26 has been selected to focus the scannedreference beam in a plane which coincides with the active surface 170 ofphotodetector 17. As a result, the cross-section of signal field 18which is analyzed or viewed by photodetector 17 is that which is also inthe plane of its active surface. Therefore, the electric signals itdevelops instantaneously represent the optical signal in field 18averaged over the area of the spot formed on the active surface of thephotodetector by reference beam 20. Since that spot is caused to bedeflected repetitively through a scanning pattern, the photodetectoroutput signals are somewhat analogous to the video signals produced byan ordinary vidicon camera. That is, the electric signal at any instantof time corresponds to the optical field characteristic at a particulartime-determined position on the scanned raster. It may be noted that,for maximum output of the electric signals from photodetector 17, thephase front of reference beam 20 must be tangential to the phase frontof signal field 18.

Being a video-type signal, the output information from photodetector 17may either be stored or recorded for subsequent use or employed in realtime to drive a television-type display device. The latter embodiment isshown in FIG. 1. For this purpose, the electric signals fromphotodetector 17 are first amplified in an amplifier and then fed to oneinput ofa mixer 31 the function of which is to convert the electricsignals to a fixed intermediate frequency f,. To that end, the systemfurther includes an IF signal generator 32 which develops a signal offrequencyf, that is summed in an adder 33 with the sweep signal offrequency f,(t) derived from generator 23. The resultant output signal,of frequencyf,(t)+f,, from adder 33 is then fed to a second input ofmixer 31 so that the modulation components on the electric signals fromphotodetector 17 now appear as modulation components on theintermediate-frequency signal at the output of mixer 31. That modulatedintermediate-frequency signal is then fed through anintermediate-frequency frequency amplifier 34 to a detector 35demodualtes the modulation components and thus converts them into actualvideo signals. These video signals are then employed in the conventionalmanner as image-forming signals in a television display receiver 37. Itsimage raster is synchronized with the scanning pattern of reference beam20 by means of a synchronizing pulse generator 38 that governs the sweepfrequency rates in both display receiver 37 and sweep frequencygenerator 23.

When detector 35 is a conventional video detector, that is, demodulatesonly the amplitude of the information content carried on theintermediate-frequency signal, display 37 develops a picture of thebrightness variations present throughout the scanning pattern in thecross-section of optical field 18 at which reference beam 20 is focused.By instead detecting phase as well as amplitude variations in themodulation, the changes in the phase throughout the displayedcrosssection of optical field 18 are reproduced in addition to thevariations in amplitude. Those phase changes are recreated on display 37as an overlying pattern of fringes. That is, display 37 produces areal-time hologram of the cross-section of signal field 18 at the focusreference beam 20. For this purpose, detector 35 is a synchronousdetector which, in addition to receiving the modulatedintermediatefrequency signal from amplifier 34, also is supplied theintermediate-frequency carrier as a reference from generator 32 througha switch 39 which is closed during that mode of detection.

The actual location of photodetector surface 17a need not coincide withthe focus of reference beam 20. That location is quite immaterial solong as the photodetector receives all of the light in both referencebeam 20 and signal field 18. The active surface may, for example, beeither to the left or the right of the focal point of beam 20 as shownin FIG. 1. This arises because, in the absence of highly dispersiveelements in the light path, photodetector l7 responds to theinstantaneous power flow in the light and that power flow is independentof position. While the total instantaneous power flow is independent ofthe position of the photo-detector, the location of photodetector 17 canaffect relative phase. That is, a change in the location ofphotodetector 17 as indicated by the dashed rectangle 17 results in thedevelopment of electric signals that are the inverse of those developedwith the photodetector located as shown in full line. This arisesbecause of the interference effects at mirror 16 which is half-silvered,that is, which passes half of the light incident thereon and reflectsthe other half.

Moreover, because photodetector 17 (or 17) responds to the instantaneouspower flow in the combined light quantities, different cross-sections ofsignal field 18 may be displayed (or stored) merely by modifying opticalsystem 25 and without photodetector 17. For example, removing lens 26results in the scanned reference beam 20 having a focus at infinity. Thedisplay on receiver 37 then is that of the far field of coherent signalfield 18. Furthermore, replacing convergent lens 25 with a divergent ornegative lens causes the scanned reference beam to diverge from avirtual focus, This still results in a display of the cross-section ofsignal field 18 in a plane which is defined by the focus of scannedreference beam 20', when the focus of reference beam 20 is virtual, theselected plane of the signal field also may be said to be virtual ratherthan real. Stated another way, photodetector l7 effectively views across-section of signal field 18 that is located a distance fromphotodetector surface 17a which is the same as is the distance of thefocus of reference beam 20 from surface 17a, and this is the caseregardless of whether that latter focus is real or virtual. As drawn inFIG. 1, the cross-section viewed is the near field at zero distance fromthe photodetector surface, since reference beam 20 is represented asbeing focused right at the photodetector surface. By instead focusingreference beam 20 at a distance from photodetector surface 17a equal tothe distance that the assumed transparency constituting object 15 islocated from that surface, the reproduced image or hologram is that ofthe optical signal field in the plane of the transparency itself.

This capability of being able, in effect, to cause photodetector 17 toview any desired cross-section of signal field 18, merely by simpleadjustment of an optical element, finds particularly advantageousutilization in observing a dynamically scattered laser beam reflectedfrom an acoustically perturbed surface. One previously disclosedapproach for reproducing acoustic holograms with the use of a scanninglaser beam is described in an article entitled Rapid Sampling ofAcoustic Holograms by Laser-Scanning Techniques by Korpel et al., whichappeared in the JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA, Volume 45,No. 4, Apr., 881-884, Apr., 1969, which technique also is the subject ofcopending application Ser. No. 763,676, filed Sept. 30, 1968 by AdrianusKorpel and assigned to the same assignee as the present application.That method and apparatus translates the acoustic field of an irradiatedsubject to a composite ripple pattern of surface-wave-perturbations on asolid surface which is then scanned with a flying spot laser scanner.That is, the perturbed surface is scanned with a focused beam ofcoherent light, and light reflected from the surface is partiallyintercepted by a knife-edge or the equivalent and fed to aphotodetector. The signal from the photodetector is applied to atelevision monitor which results in a stationary display of the originalcomposite ripple pattern on the observed surface which constitutes anacoustic hologram. A photograph taken of the television screen thenconstitutes a permanent holographic recording that may be reconstructedin a conventional way, that is, by illumination with an appropriatereference beam. Alternatively, the processing of the signals is such asto produce a conventional picture of the sound field of the surface. Thepresent system differs in that real-time holograms are obtained and thecross-section under observation may be selected as desired.

Utilizing the present system, surface wave perturbations, in themselvesproduced in the same manner, are flooded with light from laser likeobject is flooded to develop signal field 18. Turning, then, to FIG. 2,a transducer 40 generates acoustic waves represented by wavefronts 41 ata frequency f,,, determined by that of an applied electrical signal froma signal generator 42. While the acoustic wave frequency is notcritical, suitable results for many applications may be obtained byemploying an acoustic signal frequency in the range from 1 to 10Megahertz. Transducer 43 is composed of a block of suitable solidmaterial, such as methyl methacrylate (e.g., Lucite" supplied by E. l.DuPont de Nemours, lnc.), polystyrene or other plastic material providedwith a surface 44 which is rendered highly reflective by the provisionofa surface film of polished gold or the like.

Acoustically coupled to block 43 is a piezoelectric transducer 45responsive to electrical signals from generator 42 to propagate theacoustic waves toward reflective surface 44 in a direction forming anacute angle 0 with respect to normal incidence. wavefronts 41 are thusinclined at angle 0 with respect to surface 44. Accordingly, the soundwaves strike surface 44 at the angle 0, thereby causing a displacementcomponent to run upwards across surface 44 with a velocity v, which isequal to v /sin 9, where v,, is the bulk sound velocity inside block 43.Sound wave reflections are eliminated or reduced to negligible amplitudeby roughening the remaining surfaces of block 43 or lining them withsound absorbing material. Alternatively, block 43 may be so dimensionedthat the reflected wave is sufficiently attenuated by inherentabsorption in the material.

A cavity or slot 47 is machined into block 43 and filled with a liquidacoustic-wave transmission medium. The size and shape of cavity 47 arenot critical and, if desired, the construction may consist simply of arelatively thin-walled tank filled with water or other suitable liquid;in any apparatus designed specifically for use with a particular type ofobject specimen, cavity 47 preferably is formed to orient the specimenat the desired angle 0 to the acoustic wavefronts. In a preferredembodiment, block 43 is made of a methyl methacrylate plastic and slot47 is filled with water. When greater sensitivity is required, acousticimpedance matching may be provided by selection of materials and theinterposition of impedance matching layers at the interfaces between theliquid and solid media.

The object to be visualized is placed inside the cavity where itscatters the incident sound beam. Each plane wave in the angularspectrum of the scattered sound field causes its own characteristicripple pattern on the front surface 44 of the block. If the compositeripple pattern were recorded optically, by stroboscopic Schlierentechniques, it would constitute a hologram of the sound field recordedwith a fictitious reference beam incident normal to surface 44. Theattainable contrast, however, would be critically dependent on theoptical quality of the surface and has been found to be generallyinsufficient to permit direct photographic recording.

In the present system, surface 44 is flooded with a broad field of light48 which, in turn, reflects from surface 44 as an optical signal field49 to a mirror 50. Mirror 50 redirects signal field 49 through partiallytransparent mirror 16 to photodetector 17. At the same time, referencebeam 20 from scanner and shifter 22 traverses optical system 26', isredirected by mirror 27 and is in part reflected into photodetector 17by partialmirror 16. In operation, signal field 49 is perturbed byftheacoustic waves on surface 44. As already indicated, those acousticwaves, or their composite ripple pattern, constitute an acoustichologram. By suitably adjusting optical system 26 so that thecross-section viewed is at surface 44, that acoustic hologram isreproduced on display 37 in real time. To that end, a negative lens 52is interposed in the path of reference beam 20 so that the latter has avirtual focus 53 spaced the same distance along the path of beam 20 fromphotodetector surface 17a as surface 44 is spaced along the path ofsignal field 49 from the photodetector surface. If, instead of operatingdetector 35 synchronously as a phase detector, it detects onlyamplitude, display 37 produces a conventional image of surface 44.

When, on the other hand, it is desired to look at the far field of thesignal information so as, in effect, to obtain information from behindsurface 44, optical system 26 is adjusted to move focus 53 still fartherfrom photodetector l7. Illustrating an increased degree of flexibilityso as to permit selection of the viewed cross-section at any locationwithin the signal field, optical system 26" desirably takes the form ofa conventional zoom lens 55 as shown in FIG. 3.

The light of incoming signal field 48, in being reflected into signalfield 49, is diffracted by the acoustic ripple pattern on surface 44.The ripple pattern constitutes an acoustic perturbation of frequencyf,,,. This perturbation of signal field 49 causes its light to beDoppler-shifted in frequency from the original frequency fl, and fromstatically scattered light by the frequencies infm, Where n is aninteger. Accordingly, the signal from the output of mixer 31 may berepresented as including, in addition to higher order modulationcomponents corresponding to higher integers of n, f,+f,,, and f,f,,,. Byconstructing intermediate-frequency amplifier 34 to have a narrowpassband transmissive of only the desired diffraction-field component(as in a single-sideband communications receiver wherein theamplification stages include filtering selective of only a desiredsideband), the component of primary interest is permitted to be detectedwithout interference from the other components or from scattered ambientlight or noise which often is of a substantially higher intensity thanis the desired component.

In a prototype system, both the flood beam and the reference beam werederived from a helium-neon laser. Signal field power P, and localoscillator beam power l were 0.l5 milliwatts. In that case, thecross-section of both the flood and reference beams was a Zcentimetersquare. The scanning frequency, f,(t) varied from 40 Megahertz to 70Megahertz, and the intermediate frequency was either 14 Megahertz or22.5 Megahertz, depending upon the particular investigation beingconducted. Intermediate-frequency amplifier 34 had a filter bandwidth of6 Megahertz. The scanning pattern of reference beam 20, as well as thatof display 37, was operated at the standard television scanning rates of15,750 Hertz for the horizontal and 60 Hertz for the vertical.

It can be shown that the signal-to-noise ratio S/N of such a system isrepresented by the expression:

where N is the number of resolvable spots of which the scanning systemis capable, a is the sensitivity of the photodetector, e is the electroncharge, B is the filtering bandwidth, N, is the thermal noise and N,, isthe shot noise. In many cases of practical interest, P is much greaterthan P, as a result of which equation 1) becomes:

SW i I/ II) Assuming that the system is shot-noise limited, the weakestdetectable field (i.e., that field for which S/N l) is represented bythe equation:

(P,) min g (3) a In an optimally designed scanning system,

B N/2T (4) where Tis the scanning time for one picture. Sensitivity canbe increased by using a slower scan rate, keeping the value of Nconstant and reducing the bandwidth B. It can also be improved bylowering the number N of resolvable spots; if scanning speed remainsconstant, this also permits a reduction of bandwidth B.

With experimental values in the prototype system involving a bandwidthof 6 Megahertz, a resolution N of 10,000 and a photodetector sensitivityof 0.37 Amperes per watt, the theoretical minimum sensitivity given beequation (3) was 0.26 X watts. Test results in observing a signal fieldperturbed by an 85 line-per-inch Ronchi ruling afforded a visualsignal-to-noise ratio near unity when the signal power P, was lowered to1.5 X 10" watts. The experimental difference arose because thephoto-diode employed as photodetector 17 was not shot-noise limited.

Systems have thus been disclosed which find particularly advantageoususe in the filed of real-time acoustic holography and imaging. In thatutilization, the instantaneous surface displacement forms a hologram ofthe acoustic field (with the effective, although non-existent, acousticreference beam being normal to the surface). A particular cross-section,belonging to some selected order of the signal-field light diffracted bythe acoustic displacement, then becomes the reconstructed image of theacoustic field in a corresponding cross-section. By changing the optics,any desired cross-section of the optical signal field, and hence of theacoustic field, may be displayed in real time on a television screen orrecorded for alter reproduction.

lf, instead of an image of one particular acoustic cross-section, ahologram of the acoustic field is desired (as for example to be recordedfor a later visualization of any acoustic cross-section), both the phaseand amplitude of the light reflected into the selected order by theacoustic surface displacement are displayed and photographed. in thismode of operation, the system is employed as a means to produce anoptical hologram by sampling, an optical holographic techniquecorresponding to techniques previously known in acoustic holographyonly.

While particular embodiments of the present invention have been shownand described, it is apparent that changes and modifications may be madetherein without departing from the invention in its broader aspects. Theaim of the appended claims, therefore, is to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

1 claim:

1. Apparatus for analyzing an object comprising:

means for flooding said object to produce a coherent-light signal fieldperturbed by a characteristic of the object; means for developing areference beam of radiation time coherent with said light;

means for deflecting said reference beam throughout a scanning pattern;

a photodetector having a surface responsive jointly to said referencebeam and said signal field for developing electric signalsrepresentative of said characteristic;

and means for viewing a cross-section of said signal field at apre-selected distance from said photodetector surface including anoptical system for bringing said reference beam to a focus at aneffective distance from said photodetector surface corresponding to saidpre-selected distance. 2. Apparatus as defined in claim 1 which furtherincludes a display device for developing an image raster synchronouslyrelated to said scanning pattern and responsive to said electric signalfor creating on said raster a picture of said signal field at saidcross-section.

3. Apparatus as defined in claim 1 which further includes a videodetector for demodulating the amplitude of said electric signals.

4. Apparatus as defined in claim 1 which further includes a phasedetector for demodulating the phase of said electrical signal.

5. Apparatus as defined in claim 1 in which the frequency of saidradiation in said reference beam differs by a predetermined amount fromthe frequency of said light in said signal field and which furtherincludes means for mixing said electric signals with a reference signalhaving a frequency differing by said predetermined amount from anintermediate frequency and means for amplifying said intermediatefrequency.

6. Apparatus as defined in claim 1 in which the perturbation of saidsignal field creates in said electric signal a plurality of modulationcomponents individually at respective different frequencies and whichfurther includes filter means responsive to said electric signals forselecting a particular one of said modulation components.

7. Apparatus as defined in claim 1 in which said photodetector isdisposed to intercept all of the light and radiation present in bothsaid signal field and said reference beam.

8. Apparatus as defined in claim 1 in which, upon arrival at saidphotodetector surface, the phase front of said radiation in saidreference beam are tangential with the phase front of said light in saidsignal field.

9. Apparatus as defined in claim 1 in which said optical system isadjustable to vary the effective distance from said photodetectorsurface to said focus.

10. Apparatus as defined in claim 1 in which said object surfaceeffectively represents an acoustic hologram and said electric signalsinclude components representative of said acoustic hologram.

11. Apparatus for producing a signal representative of image informationin selected planes of a coherent-light-signal field spatially modulatedby an object, comprising:

a photodetector having a surface responding to light amplitudevariations to develop an electrical signal;

means for illuminating a predetermined area of said photodetectorsurface with a flood beam of coherent light spatially modulated withsaid image information, said modulated flood beam constituting saidsignal field; means for developing a reference beam of radiationtimecoherent with said light and of constant intensity;

means for scanning said reference beam over said predetermined area ofsaid photodetector surface;

and means for viewing a cross-section of said modulated flood beam at apreselected distance from said modulating object including an opticalsystem for bringing said reference beam to a focus at an effectivedistance from said object corresponding to said preselected distance sothat said signal from said photodetector varies in a mannerrepresentative of said image information over said cross-section.

1. Apparatus for analyzing an object comprising: means for flooding saidobject to produce a coherent-light signal field perturbed by acharacteristic of the object; means for developing a reference beam ofradiation time coherent with said light; means for deflecting saidreference beam throughout a scanning pattern; a photodetector having asurface responsive jointly to said reference beam and said signal fieldfor developing electric signals representative of said characteristic;and means for viewing a cross-section of said signal field at apre-selected distance from said photodetector surface including anoptical system for bringing said reference beam to a focus at aneffective distance from said photodetector surface corresponding to saidpre-selected distance.
 2. Apparatus as defined in claim 1 which furtherincludes a display device for developing an image raster synchronouslyrelated to said scanning patTern and responsive to said electric signalfor creating on said raster a picture of said signal field at saidcross-section.
 3. Apparatus as defined in claim 1 which further includesa video detector for demodulating the amplitude of said electricsignals.
 4. Apparatus as defined in claim 1 which further includes aphase detector for demodulating the phase of said electrical signal. 5.Apparatus as defined in claim 1 in which the frequency of said radiationin said reference beam differs by a predetermined amount from thefrequency of said light in said signal field and which further includesmeans for mixing said electric signals with a reference signal having afrequency differing by said predetermined amount from an intermediatefrequency and means for amplifying said intermediate frequency. 6.Apparatus as defined in claim 1 in which the perturbation of said signalfield creates in said electric signal a plurality of modulationcomponents individually at respective different frequencies and whichfurther includes filter means responsive to said electric signals forselecting a particular one of said modulation components.
 7. Apparatusas defined in claim 1 in which said photodetector is disposed tointercept all of the light and radiation present in both said signalfield and said reference beam.
 8. Apparatus as defined in claim 1 inwhich, upon arrival at said photodetector surface, the phase front ofsaid radiation in said reference beam are tangential with the phasefront of said light in said signal field.
 9. Apparatus as defined inclaim 1 in which said optical system is adjustable to vary the effectivedistance from said photodetector surface to said focus.
 10. Apparatus asdefined in claim 1 in which said object surface effectively representsan acoustic hologram and said electric signals include componentsrepresentative of said acoustic hologram.
 11. Apparatus for producing asignal representative of image information in selected planes of acoherent-light-signal field spatially modulated by an object,comprising: a photodetector having a surface responding to lightamplitude variations to develop an electrical signal; means forilluminating a predetermined area of said photodetector surface with aflood beam of coherent light spatially modulated with said imageinformation, said modulated flood beam constituting said signal field;means for developing a reference beam of radiation time-coherent withsaid light and of constant intensity; means for scanning said referencebeam over said predetermined area of said photodetector surface; andmeans for viewing a cross-section of said modulated flood beam at apreselected distance from said modulating object including an opticalsystem for bringing said reference beam to a focus at an effectivedistance from said object corresponding to said preselected distance sothat said signal from said photodetector varies in a mannerrepresentative of said image information over said cross-section.